The invention relates to video imaging camera systems.
Presently, there exist three primary encoding/decoding standards used in broadcasting color television analog signals. The NTSC (National Television Standards Committee) standard is used exclusively in North America and widely in Latin America and Japan. The PAL (Phase Alternate by Line) standard is used in the United Kingdom, most of western Europe, and Australia. Finally, the SECAM (Sequential Color with Memory) standard is used in France and eastern Europe. Each standard has its own set of specifications, each defining, for example, its own horizontal line frequency, color burst frequency, and number of lines per frame. Video cameras, whether a professional television camera or a consumer-grade camcorder, process their video signals so that their images can be viewed on a display monitor conforming to one of the above described standards. More recently, video cameras are being used as inputs for computers, where the output is a computer monitor. No interface standard has been adopted for this use.
Due to advances in electronics, the recent trend for conforming the analog signals to one of the above standards is through digital signal processing techniques. Digital signal processing generally offers higher quality video through digital storage and transmission of signals thereby preserving the integrity of the signal. Digital processing also provides more accurate control and elimination of coupling components between circuits which can break down or distort the video signals.
In some applications, processing the "raw" analog signals from the video imaging pickup device of the camera (e.g., vidicon, plumbicon, CCD) is performed, either digitally or in the analog format, at the camera itself so that the output signal from the camera can be provided directly to a display monitor. Where digital signal processing is used in an electronic camera, the analog video signals from the video pickup device are first converted to digital signals using an A/D converter. Luminance and chroma signal processing is then performed on the digital signals, which are then encoded. The digitally encoded video signals are then reconverted back, using a D/A converter, into one of the standard analog television formats (e.g., NTSC) for transmission or display on a monitor. However, in some applications, further digital signal processing, unrelated to the digital camera processing performed within the camera, is desired, for example, to compress, decompress, or provide other functions. In these applications, where digital processing is performed other than at the camera, the NTSC analog video signal is transmitted from the camera to the host for further processing, converted once again to digital data, processed, and then reconverted once again to NTSC analog video for display on a monitor. With each conversion and reconversion of the video signal comes a degradation in the quality of the signal. Moreover, where the conversion is made to a particular analog format, such as NTSC, certain information in the raw analog video signal, not needed by the NTSC receiver, is forever lost and unrecoverable. This lost information may be useful to the subsequent digital processor.
To avoid this information loss, other systems transmit from the camera the video signals in their digitally encoded format for further processing or reconversion to analog at the display monitor. In such systems, the digital video signals can be transmitted either in serial or parallel format. Transmitting the digital video signals in parallel requires a larger electrical cable with many more wires and larger connectors to interface the cable to the external processor. For example, one parallel digital bus known as SMPTE RP-125 uses a 25 pin D-type subminiature connector. Another problem with transmitting digital signals is that electrical cables longer than 20 meters generally require equalization.
To overcome the problems with parallel buses (e.g., large cables, large connectors and limited cable length) the digital video data can be transmitted over high speed serial interfaces. For example, 8 bit (240 Mbit/sec) and 10 bit (270 Mbit/sec) serial transmission interfaces have been used. In these serial interfaces 8 bit and 10 bit data words are serialized and transmitted down a standard 75.OMEGA. video coaxial cable or an optical fiber. However, serial interfaces generally require a much larger bandwidth for transmitting the video data: whereas analog NTSC requires 6 MHz, digital serial transmission of the NTSC signal requires over 100 Mbps.